Conventional technology separately recovers H2S and NH3 from sour water using an H2S stripper and an NH3 stripper, which is sometimes referred to as a two-column sour water stripping process. This process yields acid gas (H2S) with less than 50 ppmw NH3 and a high purity gaseous or liquid NH3 product. The separated water is of excellent quality, making it suitable for reuse as coke drum quench water, crude unit desalter water, and hydro-processing unit injection water or it may be sent to effluent treating for discharge.
A two-column sour water stripping process typically includes four main processing stages: 1) degassing and feed preparation; 2) H2S stripping; 3) NH3 stripping; and 4) NH3 purification and liquefaction. Referring now to FIGS. 1A and 1B, a schematic diagram of a conventional two-column sour water stripping system 100 illustrates the four processing stages. The following pressures and temperatures are exemplary and only for purposes of illustration.
Degassing and Feed Preparation:
Sour water feeds 102 from a single or several sources are combined with a recycle stream 104 from the NH3 stripper 106, which are cooled and passed through a degasser 108 where dissolved hydrogen (H2), methane (CH4) and other light hydrocarbons are removed as a hydrocarbon vapor stream 105. The sour water feeds 102 include dissolved NH3 and H2S. The recycle stream 104 includes rich NH3, which helps keep acid gases in solution in the degasser 108, thereby minimizing the release of acid gas and possible air pollution. The degassed sour water stream 109 is sent to a deoiler 103, which removes free oil from the degassed sour water stream 109 to produce a degassed/deoiled sour water stream 107. The degassed/deoiled sour water stream 107 is pumped to a feed preparation tank 110, which serves to attenuate flow rate and composition changes while also providing the opportunity to remove entrained oil and solids. The feed preparation tank 110 produces a processed sour water stream 111, which is pumped to a feed coalescer unit 112 that filters solids remaining in the processed sour water stream 111 and further separates entrained oil to produce a hydrocarbon liquid 113 and a deoiled sour water stream 115. The deoiled sour water stream 115 is sent to a feed/product exchanger 114, which acts as a heat exchanger to heat the deoiled sour water stream 115 and cool the NH3 stripper bottoms stream 132 to produce a heated deoiled sour water stream 116 and the stripped water stream 134. In this manner, the components comprising the NH3 stripper bottoms stream 132, stripped water stream 134 and the components comprising the deoiled sour water stream 115, heated deoiled sour water stream 116 are, respectively, the same but may have different concentrations and temperatures. The heated deoiled sour water stream 116 is then sent to an H2S stripper 118.
H2S Stripping:
The H2S stripper 118 contains trays or packing (not shown) that the heated deoiled sour water stream 116 flows through and around to separate H2S from the heated deoiled sour water stream 116. A cooled reflux water stream (e.g. water wash) 136 is used remove heat and suppress evolution of gaseous NH3 in the H2S stripper. A reboiler 137 acts as a heat exchanger to provide the energy required to i) heat the heated deoiled sour water stream 116 and the cooled reflux water stream 136 to a preferred temperature; and ii) strip out H2S from the heated deoiled sour water stream 116. The resulting H2S stripper overheads stream 120 is sent to a knock out drum 138 to substantially remove any entrained droplets and produce H2S stream 126. The H2S stream 126 is of high purity and is an excellent feed for a sulfur recovery unit (SRU) or a sulfuric acid plant. It contains a negligible amount of NH3 (less than 50 ppmw) and very little hydrocarbons since the sour water feeds 102 have been degassed. The H2S stream 126 is available at about 100-180 psig and 100-120° F. The resulting H2S stripper bottoms stream 130, which contains NH3 and some H2S, is sent directly to the NH3 stripper 106.
NH3 Stripping:
The NH3 stripper 106 is a steam re-boiled, refluxed distillation column. In the NH3 stripper 106, essentially all NH3 and any remaining H2S are removed from the H2S stripper bottoms stream 130, which leaves the NH3 stripper 106 as an NH3 stripper bottoms stream 132. The NW stripper bottoms stream 132 is sent to the feed/product exchanger 114 where heat is exchanged with the deoiled sour water stream 115 and the NH3 stripper bottoms stream 132 is cooled to form the stripped water stream 134. The stripped water stream 134 is suitable for many plant reuse needs or may be discharged. The containment levels of H2S and NH3 in the stripped water stream 134 may be tailored to individual requirements and is typically 10-50 ppmw NH3 and 1-25 ppmw H2S. The stripped water stream 134 is available at about 100-200° F. In the NH3 stripper 106, essentially all NH3 and any remaining H2S are removed from the H2S stripper bottoms stream 130, which leaves the NH3 stripper 106 as an NH3 stripper overheads stream 133. The NH3 stripper overheads stream 133 is sent to an overhead condenser where it is converted to an NH3 vapor stream and an NH3 liquid stream. A knock out drum 139 separates the NH3 vapor stream 140 and the NH3 liquid stream 150. A portion of the NH3 liquid stream 150 is returned as reflux to the NH3 stripper 106 and another portion of the NH3 liquid stream 150 forms the recycle stream 104. A reboiler 141 acts as a heat exchanger to provide the energy required to remove NH3 and any remaining H2S. The NH3 vapor stream 140 is an NH3-rich gas, which may be processed in a variety of ways.
NH3 Purification and Liquefaction:
Referring now to FIG. 1B, the NH3 vapor stream 140 is sent to a water wash 142 to remove residual amounts of H2S and some hydrocarbons. This step is also referred to as water scrubbing, which produces a scrubbed NH3 vapor stream 160. If NH3 recovery is not desired or economic, the scrubbed NH3 vapor stream 160 may be incinerated. In most cases, however, it is desirable to further purify the scrubbed NH3 vapor stream 160 to produce either anhydrous liquid NH3 170 or aqueous NH3 180 suitable for commercial use. In order to further purify the scrubbed NH3 vapor stream 160, the scrubbed NH3 vapor stream 160 is sent to a caustic wash 144 to remove residual contaminants including some hydrocarbons. This step is also referred to as caustic scrubbing, which produces a double scrubbed NH3 vapor stream 162 and may be necessary when problems are expected with process upsets, carbon dioxide, or complex sulfur compounds (e.g. mercaptans or disulfides). The double scrubbed NH3 vapor stream 162 may be sent to either a compressor 146 or a refrigeration unit 148 to produce the anhydrous liquid NH3 170, which contains a negligible amount of H2S (less than 5 ppmw). The anhydrous liquid NH3 170 is available at about 200 psig and 100° F. if liquefied by compression and at atmospheric pressure and about −26 F if liquefied by cooling. Cooling water and/or a refrigerant may be used to exchange heat with the compressed double scrubbed NH3 vapor stream 162. The double scrubbed NH3 vapor stream 162 may also be sent to an NH3 absorber 149, which is essentially another water wash, to produce the aqueous NH3 180, which contains a negligible amount of sulfur (no more than about 2 ppmw). The aqueous NH3 180 is available at about 35 psig and 100° F.